The present invention relates to a high voltage generating circuit for generating an anode voltage of a CRT Cathode-Ray Tube) which may be used, for instance, as a picture tube in a television receiver.
In such a high voltage generating circuit, a so-called flyback pulse which appears during horizontal blanking periods is boosted by a flyback transformer to be a high voltage pulse which is then rectified to generate a dc CRT anode voltage. Such a flyback pulse is also termed a horizontal blanking pulse.
The construction of such a prior art high voltage generating circuit is shown in FIG. 1. Horizontal driving pulses 18 derived from a horizontal driving circuit are supplied to a base of a horizontal output transistor 3 in a horizontal deflection circuit 1. Between a collector and an emitter of this transistor 3, a damper diode 4, a resonance capacitor 5 and a deflecting coil 6 are connected in parallel with each other. A capacitor 7, which is connected to the deflecting coil 6 and is intended to compensate a so-called S characteristic, operates as an effective power source for supplying a saw-toothed deflection current to the horizontal deflection coil 6. The transistor 3, the damper diode 4, the capacitors 5 and 7 and the deflecting coil 6 constitute the horizontal deflection circuit 1 in which a source voltage Vcc is supplied to a line extending to the collector of the transistor 3 through a choke coil 8.
Horizontal driving pulses 18 are also supplied to an imitative horizontal deflection circuit 2, which is substantially similar to the horizontal deflection circuit 1. This imitative horizontal deflection circuit 2 includes a dummy coil 12 corresponding to the deflecting coil, in addition to a transistor 9, a diode 10 and capacitors 11 and 13.
The line extending to the collector of the transistor 9 is connected to one terminal of a primary coil 15 of a flyback transformer 14. The secondary coils 16a and 16b of the flyback transformer 14 are connected in series through rectifying diodes 16c and 16d. Hence, the output voltages from the secondary coils are summed and supplied to an anode of a CRT (not shown). A bleeder 23 is connected to the line extending to the anode, and a voltage control circuit 20 is connected to an output terminal of the bleeder 13. This voltage control circuit 20 operates to generate a voltage which varies from the source voltage Vcc by an amount corresponding to a voltage at the output terminal of the bleeder 23, and to supply this voltage to one terminal of a primary coil 15 of the flyback transformer 14.
In addition to the primary and secondary coils 15 and 16, a tertiary coil 17 is wound on the flyback transformer 14. The output voltage from the tertiary coil 17, which is rectified by a diode and smoothed by a smoothing capacitor, is used as a low source voltage in such circuits as the horizontal driving circuit.
In the circuit of FIG. 1 constructed as mentioned above, when the horizontal output transistor 3 turns on in resonse to a horizontal driving pulse fed from the horizontal driving circuit, current will begin to flow from capacitor 7 through the deflecting coil 6, increasing linearly with time. Then, after the transistor 3 turns off, magnetic energy which has been stored in the deflecting coil 6 causes a current to flow which charges the capacitor 4, which in turn causes a voltage at a top terminal of the deflecting coil 6 to rise to a very high value. This boosted voltage appears as the first flyback pulse. When the magnetic energy in the coil 6 has been converted to electrical energy in the capacitor 5, the current flow through the deflecting coil 6 ceases. Thereafter, the electric energy stored in the capacitor is delivered through the deflecting coil 6, causing current to flow through the coil 6 in the opposite direction. After the discharge of the capacitor 5 is completed, the magnetic energy stored in the deflecting coil 6 is delivered through the damper diode 4, causing a so-called damper current to charge the capacitor 7. Then, after the transistor 3 turns on again, the same operation as mentioned above will be repeated, which causes a successive first pulse to be generated. Accordingly, repeating the operation, a saw-toothed current flows through the deflecting coil 6.
The imitative horizontal deflecting circuit 2 operates in the same manner as the horizontal deflecting circuit 1 such that a voltage at a top terminal of a dummy coil 12 is raised to a very high value, causing the generation of the second pulse synchronous with the first pulse. This second pulse is fed to the primary coil 15 of the flyback transformer 14 as a flyback pulse, which causes a boosted voltage to appear across the secondary coils 16a and 16b, which is fed to the anode after being rectified by diodes 16c and 16d.
The peak voltage V.sub.p of this flyback pulse is given by: EQU V.sub.p =V.sub.B {2.pi.(t.sub.H /.sqroot.L.sub.D .multidot.C.sub.r -1)+1}, (1)
where V.sub.B is the dc voltage supplied from the voltage control circuit 20, L.sub.D is the reactance of the dummy coil 12, C.sub.r is the capacitance of the capacitor 11, and t.sub.H is the horizontal deflection time (63.5 .mu.S).
The anode current of a CRT varies with changes in load due to a change in brightness of the screen such that the anode current decreases with the brightness of the screen. Hence, the high voltage output supplied to the anode varies with the brightness of the screen. As can be understood from equation (1), since the peak voltage V.sub.p of the flyback pulse varies with the voltage V.sub.B, the high voltage output can be stabilized by changing the peak voltage V.sub.p of the flyback pulse through control of the voltage V.sub.B.
Hence, in the high voltage generating circuit according to the prior art, the voltage control circuit 20 operates such that it produces a dc voltage V.sub.B which varies from the source voltage Vcc by an amount corresponding to the voltage at the output terminal of the bleeder 23 monitoring the anode voltage. This dc voltage V.sub.B is supplied to the imitative horizontal deflection circuit 2 through the primary coil 15 of the flyback transformer 14.
However, in this conventional high voltage generating circuit, the peak voltage V.sub.p must be varied by changing the dc voltage V.sub.B in order to stabilize the high voltage output, i.e., the anode voltage, and hence the low voltage output induced across the tertiary coil 17 also varies. Therefore, a problem arises in that an additional device such as a voltage regulator is required to stabilize this low voltage output. In addition, another problem arises in that an expensive choke coil is also to be required.
Furthermore, two horizontal output transistors are required in both the horizontal deflection circuit and the imitative horizontal deflection circuit. Hence, a relatively large space is required in this circuit considering that heat radiating fins must usually be provided for these transistors to facilitate heat dispersion from them. Accordingly, the prior art circuit utilizes space poorly and requires many steps in manufacture.
Still further, a current signal, which has a level of only several microamps to several milliamps, used for automatic brightness limiting (ABL) cannot be picked up from the secondary side of the flyback transformer.